The present invention relates generally to improvements in injection valves and, more particularly, but not by way of limitation, to valves used to inject solvents into conduits of coal deashing plants.
The process for deashing coal by the treatment of pulverized coal with a solvent at elevated temperature and pressure has been described in U.S. Pat. No. 4,162,956 issued July 31, 1979 to Donald E. Rhodes, such patent being assigned to the assignee of the present invention. In this process, a solvent-coal mixture is permitted to separate into two phases in a separation zone with the heavier phase, comprised of a suspension of ash in the solvent, gravitating to the lower end of the vessel defining the zone. This suspension, or slurry, is drawn from the vessel and transported to a flash zone wherein the slurry is flashed to recover the solvent which can then be reused in the deashing process.
As will be recognized by those skilled in the art, the transport of the slurry to the flashing zone can present practical problems that must be overcome if the operation of the deashing plant is to be successful. In particular, the danger exists that the ash will settle from the slurry during transport to the flash zone to plug the conduit by means of which the transport is effected. Should this occur, the process must be interrupted and the plant partially dismantled to eliminate the blockage. This is neither a simple nor inexpensive task because of the nature of the materials involved and the temperature and pressure conditions under which the process is carried out. Moreover, these problems feed upon themselves; the conditions in the conduit which give rise to the difficulties of clearing a blockage are necessary to maintain the ash in suspension.
A variety of factors enter into the formation of a blockage in the conduit to the flash zone and one such factor is the consistency of the slurry. The tendency of the ash to settle out from the slurry increases with the weight of the slurry and, because of this factor, it is useful to provide the conduit with a means for injecting additional solvent into the conduit should the slurry, as drawn from the separation zone, contain too high a proportion of ash to solvent to be transported to the flash zone without risk of a blockage occurring. Because of the conditions of operation of a coal deashing plant, the injection of the additional solvent must be under the control of the plant operator and, accordingly, such injection is carried out through an injection valve provided for that purpose.
A problem has occurred with this aspect of deashing plant operation and such problem stems from other factors that enter into the separation of the ash from the solvent with which it forms a slurry. In particular, the ash tends to form deposits on surfaces in which it is in static contact and also tends to form deposits at cold spots in the conduit-injection valve system. While these conditions are prevented in the conduit itself; for example, cold spots are prevented by providing the conduit with a concentric insulating jacket, the conditions can occur in portions of the valve in fluid communication with the flow passage through the conduit. When the valve is closed, the slurry can enter the port of the valve through which the solvent is injected into the conduit and stagnate therein. In addition, the valve itself forms a heat leak from the conduit so that the valve will have a tendency to cool slurry that has entered portions of the valve. The immediate result is that the valve exit port can become plugged so that additional solvent cannot be injected into the conduit when such additional solvent is needed. The eventual result can be a blockage of the conduit itself that requires an expensive shut down of the plant to clear ash deposits from the valve and from the conduit to the flash zone.